1. Technical Field
The present disclosure relates to an improved detection structure for a resonant accelerometer with vertical axis (so-called “z-axis”), of a MEMS (microelectromechanical system) or NEMS (nanoelectromechanical system) type, in particular one capable of detecting with high electrical performance a component of vertical acceleration, acting in a direction transverse with respect to, or out of, a plane of a main extension of the same structure.
2. Description of the Related Art
As is known, accelerometers of a MEMS (or NEMS) type have been proposed and used, thanks to their extremely compact dimensions, low consumption levels, and good electrical performance, for a wide range of fields of application, for example for inertial navigation applications.
The various accelerometers proposed in the literature and currently present on the market may be generally grouped into three classes, based on the principle of detection used by the corresponding detection structure: capacitive accelerometers, piezoresistive accelerometers, and resonant accelerometers.
In resonant accelerometers, the external acceleration to be measured produces a detectable shift of the resonance frequency of one or more resonator elements of the mechanical detection structure; the resonator element can be constituted by an entire inertial mass (free mass or proof mass) of the detection structure, or by some part thereof. Resonant detection, as compared to other measuring principles, has the advantage of offering a direct frequency output, of a quasi-digital type, high-sensitivity, and wide-dynamic-range.
According to the configuration of the detection structure, the variation of resonance frequency may be induced by the presence of axial stresses in the resonator element or by the variation of the so-called “electrical stiffness” to which the same resonator element is subjected.
In particular, z-axis resonant accelerometers have been proposed, made using the “surface micromachining” technique, whose operating principle is based on the detection of a resonance frequency variation due to a variation of electrical stiffness.
For instance, accelerometers of this sort are described in the following documents:
[1] S. Sung, J. G. Lee, T. Kang, “Development and test of MEMS accelerometer with self-sustained oscillation loop”, Sensors and Actuators, 109, 1-8 (2003);
[2] B. Lee, C. Oh, S. Lee, Y. Oh, K. Chun, “A vacuum packaged differential resonant accelerometer using gap sensitive electrostatic stiffness changing effect”, Proc. MEMS 2000;
[3] H. C. Kim, S. Seok, I. Kim, S-D. Choi, K. Chun, “Inertial-grade out-of-plane and in-plane differential resonant silicon accelerometers (DRXLs)”, Proc. Transducers '05, Seoul, Korea, Jun. 5-9, 172-175 (2005).
The operating principle of these resonant accelerometers may be summarized as follows: an external acceleration a generates on an inertial mass m of the detection structure an inertial force F=m·a; this external force induces a displacement or a rotation of the inertial mass, proportional thereto, which causes variation of the distance or gap between the inertial mass and the substrate facing it (and detection electrodes provided on the same substrate). The gap variation produces a variation of electrical stiffness Ke, and this causes a corresponding variation of the resonance frequency of the resonating element, whether this is constituted by the entire inertial mass, by a part thereof, or by a distinct element coupled thereto.
In particular, the accelerometer based upon this principle proposed in document [1] uses as resonator element the entire inertial mass of the detection structure, suitably suspended above the substrate by means of elastic supporting elements, set at the edges of the same inertial mass, and appropriately kept in a condition of resonance by an electronics coupled thereto.
This structure has, however, the disadvantage that, since the detection axis coincides with the axis of oscillation of the resonant mass, it is difficult to check whether the resonant mode is stable; moreover, given the dimensions of the resonant mass (which corresponds to the entire inertial mass), the amount of energy required to drive it in resonance may in general be high.
An alternative solution is represented by the accelerometers proposed in the documents designated previously by [2] and [3], where the microelectromechanical detection structure is constituted by an inertial mass and by two torsional resonators coupled thereto. The inertial mass is constrained to the substrate eccentrically, and is set in rotation about a rotation axis in the presence of an external acceleration; the torsional resonators have an axis of rotation of their own, orthogonal to that of the inertial mass, and are separately kept in resonance. The displacement of the inertial mass causes variation of the electrical stiffness felt by the resonators, and hence a variation of the resonance frequency thereof.
The sensitivity reported in the literature for resonant accelerometers made by surface micromachining are of a few tens of hertz for 1 g of acceleration. For example, in the case of the device described in document [2], the sensitivity reaches approximately 70 Hz/g with overall dimensions of the mobile inertial mass of approximately 2.5 mm×2 mm, with a thickness of 40 μm (i.e., dimensions that are rather large, above all in the case of portable applications). In the case of the device described in document [1], the sensitivity reaches approximately 25 Hz/g with overall dimensions of the mobile inertial mass of approximately 1 mm×1 mm, with a thickness of 40 μm.
The various resonant MEMS accelerometers so far proposed hence differ from the standpoint of the arrangements provided for the mechanical detection structure (in particular, from the standpoint of the different arrangements of the resonator elements with respect to the inertial mass), and consequently from the standpoint of the electrical characteristics that derive therefrom, in particular as regards the detection sensitivity to the external acceleration.
None of these accelerometers is, however, completely satisfactory as regards the electrical characteristics and mechanical dimensions, mainly in the case of portable applications in which particularly low consumption levels and small dimensions are required.